Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field (B0 field) whose direction at the same time defines an axis (normally the z-axis) of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength. These energy levels can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as B1 field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°).
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of one or more receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The MR signal data obtained via the RF coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation.
Known parallel acquisition techniques are used widely in MR imaging for accelerating the MR signal acquisition. A method in this category is SENSE (Sensitivity Encoding). SENSE and other parallel acquisition techniques use undersampled k-space data acquisition obtained from multiple RF receiving antennas in parallel, wherein the RF receiving antennas have different spatial sensitivity profiles. In these methods, the (complex) signal data from the multiple RF receiving antennas are combined with complex weightings in such a way as to suppress undersampling artifacts (aliasing) in the finally reconstructed MR images. This type of complex RF coil array signal combination is sometimes referred to as spatial filtering and includes combining in the k-space domain or in the image domain (in SENSE), as well as methods which are hybrids.
In SENSE imaging, coil sensitivity profiles are typically estimated from low-resolution reference data obtained by a SENSE reference scan. This coil sensitivity information is then used during image reconstruction to “unwrap” aliased pixels in image space using a direct inversion algorithm.
Conventionally, the MR device employed for a given diagnostic imaging task automatically detects when a SENSE reference scan is required depending on the type and the parameters of the selected imaging sequence. The SENSE reference scan is automatically inserted into the list of sequences to be performed, typically immediately before the diagnostic imaging sequence.
The SENSE reference scan usually includes two scans because MR signal data have to be acquired (i) via the multiple RF receiving antennas of which the spatial sensitivity profiles are to be determined and (ii) via a body RF coil having an essentially homogeneous spatial sensitivity profile as a reference. These two scans have to be performed separately for the reason of decoupling the array of RF receiving antennas from the body RF coil.
Moreover, an accurate measurement of the spatial distribution of the transmitted RF field is often important for many MR imaging applications (particularly at high main magnetic field strengths of 3 Tesla or more) to support appropriate prospective (if applicable) and retrospective correction/compensation. This requires a robust and fast B1 mapping technique. However, most B1 mapping techniques are relatively slow, making integration into the clinical workflow difficult. The international application WO2013/05006 mentions formation of a B1 map indicating the spatial distribution of the RF field of the RF pulses within the portion of the body (10) from acquired FID and stimulated echo signals.
Sometimes, also the distribution of the main magnetic field B0 needs to be determined in a preparation scan prior to the actual diagnostic scan. This enables B0 shimming and/or compensation of B0 inhomogeneities during MR image reconstruction.
All these different preparation scans significantly increase the total scan time.